Zinc anode for electrochemical cells

ABSTRACT

An additive compound for oxidizable metal such as zinc is provided. The additive includes a sorbitan based compound of the formula:  
                 
 
     wherein R1, R2, R3 may be the same or different, and are each selected from the group consisting of OH and (OCX 1 X 2 CX 3 X 4 ) n OH, where X 1  X 2 , X 3 , X 4  are selected from the group consisting of H, F, and an aliphatic group, wherein n is between 1 and about 10000;  
     R4 is selected from the group consisting of a single bond, OH and (OCX 1 X 2 CX 3 X 4 ) n OH, where X 1  X 2 , X 3 , X 4  are selected from the group consisting of H, F, and an aliphatic group, wherein n is between 1 and about 10000; and  
     R5 is selected from the group consisting of OR6 and OOCR6, wherein R6 is an aliphatic group.

RELATED APPLICATIONS

[0001] The present application claims priority to Unites StatesProvisional Patent Application Ser. No. 60/298,537 entitled “ZINC ANODEFOR ELECTROCHEMICAL CELLS” filed on Jun. 15, 2001 by the inventorsherein, the entire disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field Of The Invention

[0003] This invention relates to zinc anode for electrochemical cells,and particularly to corrosion inhibiting anode materials.

[0004] 2. Description Of The Prior Art

[0005] Zinc and zinc alloys have been used for many years as activematerials in electrochemical cells, including zinc air, zinc-silver,zinc-manganese, zinc nickel, zinc halide, and other cell systems. Zincis preferred in many applications because of the relatively high energydensities, as well as its natural abundance. However, one associatedproblem with zinc-based electrochemical cells relates to self discharge,generally wherein hydrogen gasses from the system upon self -dischargeof the cell. This effect, generally referred to as anode corrosion inmany cell systems, detriments the life of the cell, as zinc is consumedfor the production of unwanted hydrogen gas rather than energy. Further,such gas evolution will increase the cell internal pressure, which maylead to leakage of the electrolyte.

[0006] Previous attempts at suppressing possible corrosion of the zincpowder as an anode active material employed mercury-containing zincalloys. Certain mercury-containing zinc effectively maintained anacceptable storing property of a cell having such an anode. However, dueto environmental concerns, decreasing mercury content in the anode zincalloy powder and commercialization of a battery including anon-amalgamated, mercury free zinc alloy powder have been demanded inrecent years.

[0007] Typical approaches included incorporating any of bismuth,aluminum, calcium, indium, tin, and other materials to impartcorrosion-resistant properties and suppress the generation of gas due tothe corrosion of the zinc powder. This zinc alloy powder is thus takenas a promising anode zinc material for the mercury free alkalinebattery. However, oftentimes, these various alloy components stillresult in a significant amount of hydrogen evolution during the extendedstorage of the cell.

[0008] Other approaches to varying anode material by surface treatmentof the anode with the certain polymers or surface active agents, oralternatively electrolytes, have been attempted, however, most generallyresult in similar outcomes. That is, while corrosion reduction has beenpossible, heretofore the discharge capacity of such corrosion systems islacking.

[0009] Accordingly, it would be desirable to provide an anti-corrosionzinc anode material while maintaining suitable discharge capacities,particularly at high discharging rate.

SUMMARY OF THE INVENTION

[0010] The above-discussed and other problems and deficiencies of theprior art are overcome or alleviated by the several compositions,wherein a zinc material is coated with an inhibitor compound. Theinhibitor compound generally comprises a surfactant having the generalformula:

[0011] wherein R1, R2, R3 may be the same or different, and are eachselected from the group consisting of OH and (OCX₁X₂CX₃X₄)_(n)OH, whereX₁ X₂, X₃, X₄ are selected from the group consisting of H, F, and analiphatic group such as CH₃ or CH₃CH₂) (e.g., (OCH₂CH₂)_(n)OH), whereinn is between 1 and about 10000;

[0012] R4 is selected from the group consisting of a single bond, OH and(OCX₁X₂CX₃X₄)_(n)OH, where X₁ X₂, X₃, X₄ are selected from the groupconsisting of H, F, and an aliphatic group such as CH₃ or CH₃CH₂) (e.g.,(OCH₂CH₂)_(n)OH), wherein n is between 1 and about 10000; and

[0013] R5 is selected from the group consisting of OR6 and OOCR6,wherein R6 is an aliphatic group.

[0014] The above-discussed and other features and advantages of thepresent invention will be appreciated and understood by those skilled inthe art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a representation of discharge capacity data for variouselectrochemical cells using different anode materials including a zincmaterial having an inhibitor compound therein as described herein.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0016] An inhibitor compound for use with a zinc based electrodematerial generally comprises a sorbitan based compound of the generalformula:

[0017] In formula (1), R1, R2, R3 may be the same or different, and areeach selected from the group consisting of OH and (OCX₁X₂CX₃X₄)_(n)OH,where X₁ X₂, X₃, X₄ are selected from the group consisting of H, F, andan aliphatic group such as CH₃ or CH₃CH₂) (e.g., (OCH₂CH₂)_(n)OH),wherein the values for n (hereinafter n_(R1), n_(R2), n_(R3)) aregenerally each between 1 and about 10000. R4 is selected from the groupconsisting of a single bond, OH and (OCX₁X₂CX₃X₄)_(n)OH, where X₁ X₂,X₃, X₄ are selected from the group consisting of H, F, and an aliphaticgroup such as CH₃ or CH₃CH₂) (e.g., (OCH₂CH₂)_(n)OH), wherein n(hereinafter n_(R4)) is between 1 and about 10000. In a preferredembodiment, the SUM Of n_(R1), n_(R2), n_(R3,) and n_(R4) is less thanabout 10000, more preferably less than about 1000, and most preferablyless than about 200. Further, R5 is generally an ester compound or anether compound, selected from the group consisting of −OR6 and −OOCR6,wherein R6 is an aliphatic group.

[0018] Examples of compounds of the general formula (1) useful asinhibitor compounds include, but are not limited to: polyoxyethylenesorbitanaliphatic acid esters such as polyoxyethylene sorbitanmonolaurate, polyoxyethylene sorbitan monopalmitate, polyoxyethylenesorbitan monostearate, polyoxyethylene sorbitan monooleate,polyoxyethylene sorbitan trioleate, and polyoxyethylene sorbitantristearate; and: sorbitanaliphatic acid esters such as sorbitanmonolaurate, sorbitan monopalmitate, sorbitan monostearate, sorbitanmonooleate, sorbitan trioleate, and sorbitan tristearate. However, oneof skill in the art will appreciate that other sorbitan based compoundsof the general formula (1) may be employed. Further, variouscombinations of inhibiting compounds of the formula (1) may be employed.

[0019] The inhibitor compound may be incorporated in a zinc or zincalloy anode material or an electrochemical cell based on such zinc anodematerial in several ways. For example, the inhibitor compound may beadded to an electrolyte, which may be gelled and mixed with the zincmaterial. Alternatively, the inhibitor compound can be added directly tothe zinc anode material. For example, the inhibitor compound may bedissolved in a suitable solvent such as acetone or water to form adiluted solution. The zinc material may be soaked in the solution,whereby the solvent is subsequently evaporated at room temperature or anelevated temperature such as 50° C. Alternatively, the diluted inhibitorcompound solution may be sprayed on the zinc material. Further, the zincmaterial may be tumbled with a solution of the inhibitor compound andsubsequently dried. Regardless of the method employed, the inhibitorcompound is preferably distributed homogeneously throughout the zincmaterial to provide uniform corrosion resistance.

[0020] The inhibitor compound is provided in an amount that issufficient to inhibit or substantially prevent the occurrence of anodecorrosion. In general, amounts of about 1 part per million (ppm) toabout 5% may be employed, preferably about 50 ppm to about 2000 ppm,more preferably about 50 ppm to about 200 ppm with respect to the weightof zinc material. It will be appreciated from one skilled in the artthat the actual amount of the inhibitor compound necessary may bedetermined by well-known methods.

[0021] The zinc material may comprise zinc metal. Zinc may also bealloyed with constituents including, but not limited to, bismuth,calcium, magnesium, aluminum, lithium, indium, lead, mercury, gallium,tin, cadmium, germanium, antimony, selenium, thallium, or combinationscomprising at least one of the foregoing constituents. The metalconstituent may be provided in the form of foil, powder, dust, granules,flakes, needles, pellets, fibers, or other particles.

[0022] The zinc material coated with the inhibitor compound may beemployed as a dry material, i.e., without electrolyte. Such material isuseful, for example, in electrochemical cell systems where theelectrolyte is provided independent of the zinc material.

[0023] Alternatively, the zinc material may be incorporated with anelectrolyte, for example, to form a zinc paste. Suitable electrolytematerials include ion conducting material to allow ionic conductionbetween the metal anode and the cathode. An ion conducting amount ofelectrolyte may be provided in the anode material. The electrolytegenerally comprises ionic conducting materials such as KOH, NaOH, LiOH,other materials, or a combination comprising at least one of theforegoing electrolyte media. Particularly, the electrolyte may compriseaqueous electrolytes having a concentration of about 5% ionic conductingmaterials to about 55% ionic conducting materials, preferably about 10%ionic conducting materials to about 55% ionic conducting materials, andmore preferably about 35% ionic conducting materials to about 45% ionicconducting materials.

[0024] A gelling agent may also be used in sufficient quantity toprovide the desired consistency of the paste. The percentage of gellingagent (based on the total electrolyte without zinc material) isgenerally about 0.2% to about 20%, preferably about 1% to about 10%,more preferably about 1% to about 5%. The gelling agent may be acrosslinked polyacrylic acid (PAA), such as the Carbopol® family ofcrosslinked polyacrylic acids (e.g., Carbopol® 675, Carbopol® 940)available from Goodrich Corp., Charlotte, N.C., and potassium and sodiumsalts of polyacrylic acid or polymethyl acrylic acid; carboxymethylcellulose sodium salt (CMC), such as those available from AldrichChemical Co., Inc., Milwaukee, Wis.; hydroxypropylmethyl cellulose;polyvinyl alcohol (PVA); poly(ethylene oxide) (PEO); polybutylvinylalcohol (PBVA); natural gum; Polygel 4P (available from Sigma-Aldrich);grafted starch, such as Waterlock® A221, available from Grain ProcessingCorp., Muscatine, Iowa.; combinations comprising at least one of theforegoing gelling agents; and the like.

[0025] Further, the zinc paste may include zinc oxide, generally toprovide further reduction in gassing of the material. The zinc oxide maybe included in the electrolyte or in the zinc or zinc alloy material.

[0026] The invention will now be described by way of a non-limitingexample.

Example

[0027] A zinc material having an embodiment of the inhibitor compoundwas prepared as follows. A zinc alloy was provided having the followingassay: 102 ppm Bi 200 ppm In  99 ppm Al  33 ppm Pb (unavoidableimpurity)  2 ppm Fe (unavoidable impurity)  5 ppm Cu (unavoidableimpurity)  1 ppm Cd (unavoidable impurity)

[0028] The zinc alloy further had a particle distribution as follows:+425 micrometers  0% +250 micrometers 23% +150 micrometers 37% +106micrometers 24%  +75 micrometers 15%  −75 micrometers  2%

[0029] The zinc alloy was tumbled with a 1% solution of an inhibitorcompound comprising polyoxyethylene(20) sorbitan monooleate in water.The tumbled mixture was dried at 50° C. until substantially all of thesolvent evaporated. Overall, about 0.01 wt. %, based on the weight ofthe zinc alloy, of the inhibitor compound (without solvent) was providedto coat the zinc alloy.

[0030] Gassing tests were performed with 5 grams the zinc materialincorporating the inhibitor compound with 10 milliliters of a 40% KOHsaturated with zinc oxide solution. The test demonstrated that the anodematerial incorporating the inhibitor compound did not gas after onemonth at 50° C. Further, at 70° C., gassing tests demonstrated that theanode material incorporating the inhibitor compound evolved less than 3to about 4 microliters per day for seven days.

[0031] To verify discharging capacities of electrochemical cells formedwith the anode material incorporating the inhibitor compound, a cell wasconstructed employing the anode material in combination with anelectrolyte material. The electrolyte material comprised about 1.8%Carbopol 675 (based on the total amount of the electrolyte material) ina 45% KOH aqueous solution. Additionally, 4% zinc oxide was incorporatedinto the electrolyte material.

[0032] The positive electrode used for each of the cells (using the zincanode material and using comparative anode materials) of a test cellcomprised an air diffusion cathode having a cobalttetramethoxyphenylporphyrin (CoTMPP) catalyst supported on carbon withTeflon® binder. The air diffusion cathode was optimized for 100 mA/cm²discharge current density. Nickel sponge current collectors were used inelectrical contact with the cathodes, and 2 millimeter thick copper foilcurrent collectors were used for the anode.

[0033] Comparative cells were formed from anode material obtained fromexisting D-sized batteries. These batteries included Energizer®(consumer), Energizer® (industrial), Energizer® Titanium e², Duracell®,and Rayovac® Maximum™.

[0034] Two cell were assembled using a 2 millimeter thick layer of anodepaste applied to a 3 cm by 5 cm cathode and separator (FreudenbergFS2213E non woven nylon separator). A cell was constructed using theabove described anode paste comprising polyoxyethylene(20) sorbitanmonooleate as an inhibitor compound. Likewise, anode material from theexisting batteries was also used to form the comparative cells.

[0035] The cells were all discharged at 1.5 Amps constant current. Ingeneral, the cells formed from the anode material described hereinoutperformed the cells formed from the anode material from the existingbatteries. FIG. 1 provides the discharging times for the various cells(two tests per cell in the example).

[0036] As can be seen from the FIG. 1, the discharge capacity of thepresent anode material including the inhibitor compound is increased byabout 30% to up to about 45% as compared to anode material from existingcells. This benefit was achieved while minimizing corrosion and gassingof the cell due to unwanted reaction of the anode material.

[0037] Various benefits may be derived from the zinc material andelectrochemical cells using the zinc material. Particularly, the anodematerial minimizes or eliminates self discharge and other detrimentsassociated with gassing of zinc material. Further, the dischargingcapacities of the anode materials described herein are improved ascompared to existing anode materials. Further, the inhibitor compoundmay also be used as additives for other oxidizable metal anodes.

[0038] While preferred embodiments have been shown and described,various modifications and substitutions may be made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustrations and not limitation.

What is claimed is:
 1. An additive compound for oxidizable metalscomprising a sorbitan based compound of the formula:

wherein R1, R2, R3 may be the same or different, and are each selectedfrom the group consisting of OH and (OCX₁X₂CX₃X₄)_(n)OH, where X₁ X₂,X₃, X₄ are selected from the group consisting of H, F, and an aliphaticgroup, wherein n is between 1 and about 10000; R4 is selected from thegroup consisting of a single bond, OH and (OCX₁X₂CX₃X₄)_(n)OH, where X₁X₂, X₃, X₄ are selected from the group consisting of H, F, and analiphatic group, wherein n is between 1 and about 10000; and R5 isselected from the group consisting of OR6 and OOCR6, wherein R6 is analiphatic group.
 2. A zinc anode material comprising zinc and theadditive compound of claim
 1. 3. The zinc anode material as in claim 2wherein additive compound comprises about 1 ppm to about 5%, on a weightbasis, with respect to the
 4. The zinc anode material as in claim 2wherein additive compound comprises about 50 ppm to about 2000 ppm, on aweight basis, with respect to the zinc.
 5. The zinc anode material as inclaim 2 wherein additive compound comprises about 50 ppm to about 200ppm, on a weight basis, with respect to the zinc.
 6. The zinc anodematerial as in claim 2, wherein compound (1) is selected from the groupconsisting of polyoxyethylene sorbitanaliphatic acid esters such aspolyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitanmonopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylenesorbitan monooleate, polyoxyethylene sorbitan trioleate, andpolyoxyethylene sorbitan tristearate.
 7. The zinc anode material as inclaim 2, further comprising an alloy constituents selected from thegroup consisting of bismuth, calcium, magnesium, aluminum, lithium,indium, lead, mercury, gallium, tin, cadmium, germanium, antimony,selenium, thallium, and a combination comprising at least one of theforegoing alloy constituents.
 8. The zinc anode material as in claim 2,further comprising an electrolyte.
 9. The zinc anode material as inclaim 8, wherein the electrolyte comprises ionic conducting materialselected from the group consisting of KOH, NaOH, LiOH, and a combinationcomprising at least one of the foregoing ionic conducting materials. 10.The zinc anode material as in claim 8, further comprising a gellingagent.
 11. In an electrochemical cell having a zinc anode and a cathodein substantially electrical isolation and in ionic communication, theimprovement comprising an amount of the inhibitor compound of claim 1 insaid zinc anode.